It is common knowledge in the food packaging industry that after a container is filled with certain foods and is closed, the container and its content must be thermally processed to sterilize the food so that it will be safe for human consumption.
Thermal processing of such containers is normally carried out at temperatures higher than about 190.degree. F. in various equipment such as rotary continuous cookers, still retorts and the like, and the containers are subjected to various cook-cool cycles before they are discharged, stacked and packed for shipment and distribution. Under these thermal processing conditions, plastic containers tend to become distorted or deformed due to sidewall panelling (buckling of the container sidewall) and/or distortion of the container bottom wall, sometimes referred to as "bulging" or "rocker bottom". These deformations and distortions are unsightly, and interfere with proper stacking of the containers during their shipment, and also cause them to rock and to be unstable when placed on counters or table tops. In addition, bottom bulging is, at times, considered to be a possible indication of spoilage of the food thus resulting in the rejection of such containers by consumers.
One reason for the distortion of the container is that during thermal processing the pressure within the container exceeds the external pressure, i.e., the pressure in the equipment in which such process is carried out. One solution to this problem is to assure that the external pressure always exceeds the internal pressure. The conventional means of achieving this condition is to process the filled container in a water medium with an overpressure of air sufficient to compensate for the internal pressure. This is the means used to process foods packed in glass jars and in the well-known "retort pouch". The chief disadvantage of this solution is that heat transfer in a water medium is not as efficient as heat transfer in a steam atmosphere. If one attempts to increase the external pressure in a steam retort by adding air to the steam, the heat transfer efficiency will also be reduced relative to that in pure steam.
Several factors contribute to the increase in internal pressure within the container. After the container is filled with food and hermetically closed, as a practical matter, a small amount of air or other gases will be present in the headspace above the food level in the container. This headspace of air or gas is present even when the container is sealed under partial vacuum, in the presence of steam (flushing the container top with steam prior to closing) or under hot fill conditions (190.degree. F.). When the container is heated during thermal processing, the headspace gases undergo significant increases in volume and pressure. Additional internal pressures will also develop due to thermal expansion of the product, increased vapor pressures of the products, the dissolved gases present within the product and the gases generated by chemical reactions in the product during its cooking cycle. Thus, the total internal pressure within the container during thermal processing is the sum total of all of the aforementioned pressures. When this pressure exceeds the external pressure, the container will be distorted outwardly tending to expand the gases in the headspace thereby reducing the pressure differential. When the container is being cooled, the pressure within the container will decrease. Consequently, the sidewall and/or the bottom wall of the container will be distended inwardly to compensate for the reduction in pressure.
It has been generally observed that such thermally processed plastic containers may remain distorted because of bulging in the bottom wall and/or sidewall panelling. Unless these deformities can be eliminated, or substantially reduced, such containers are unacceptable to consumers.
It must also be noted that it is possible to make a container from a highly rigid resin with sufficient thickness to withstand the pressures developed during thermal processing and thus alleviate the problems associated therewith. However, practical considerations and economy militate against the use of such containers for food packaging.
Accordingly, it is an object of this invention to improve the configuration of a plastic container after thermal processing.
It is another object of this invention to alleviate the problems associated with bottom bulging and sidewall panelling of a plastic container which result from thermal processing.
It is a further object of this invention to attain an acceptable container configuration after such container is packed with food, hermetically closed and thermally processed.
It is still another object of this invention to provide methods, and container configurations which permit plastic containers to have acceptable configurations despite their having been subjected to thermal food processing conditions.
It is yet another object of this invention to facilitate thermal food processing of plastic containers packed with food.
The foregoing and other objects, features and advantages of this invention will be further appreciated from the ensuing detailed description and the accompanying drawings.